In addition to conventional read-only optical disks such as CD-ROM, CD-R, and DVD-ROM, there has been increasing demand in recent years for writable optical disks, such as CD-R/RW and DVD-RAM, which are primarily used as personal computer appliances. In a recording and reproducing device of such writable optical disks, the quantity of irradiated light is much larger in writing than in reproducing, and accordingly a photo-detector amplifier circuit, which receives reflected light off the optical disk, has a large light input. In order to accommodate a difference in light input level between writing and reproducing, the photo-detector amplifier circuit adopts a system in which gains are switched between writing and reproducing.
FIG. 10 is a block diagram showing an electrical structure of a common recording and reproducing device 1. A signal from a signal source 2 enters an LD driving circuit 3 that is provided with a semiconductor laser. A beam splitter 4 splits an optical signal from the semiconductor laser for entry into a photo-detector element A2 and an optical disk 5. The light that was projected on the optical disk 5 and reflected off the disk surface enters the photo-detector element A1 via the beam splitter 4 and is converted and amplified into an electrical signal before it is supplied to a second-stage circuit where signal reproduction is carried out. The photo-detector element A1 also outputs a signal that enters a laser power control circuit 6 with an output signal from a photo-detector element A2, so as to adjust output laser power.
FIG. 11 is a block diagram showing an electrical structure of a photo-detector amplifier circuit 10 of a typical conventional example in the recording and reproducing device 1, in which gains are switched between writing and reproducing. The photo-detector element A1 is made up of a photodiode pd that is divided into four channel areas A through D. The photo-detector amplifier circuit 10 includes a first-stage amplifier 11 and a second-stage amplifier 12 that are individually provided for the channels A through D, and a second-stage amplifier 13 that is commonly provided for the channels A through D. The optical signal of the reflected light from the optical disk 5 is converted into a current signal isc in the photodiode pd. The current signal isc is subjected to current-voltage conversion and amplified in an amplifier a11.
The output of the amplifier a11 of the first-stage amplifier 11 of each channel A to D is supplied to a non-inverted terminal of a differential amplifier a21 of the corresponding second-stage amplifier 12 and to a non-inverted terminal of a differential amplifier a22 of the common second-stage amplifier 13 that is provided for collection (addition) (described later). In the differential amplifiers a21 and a22, the input is compared with their respective reference voltages before it is amplified and outputted. The output signal of the differential amplifier a21 is used for focusing and tracking servo of the optical system. The output signal of the differential amplifier a22 is used to read out signal information written in the disk.
Switching gains between writing and reproducing is carried out by the amplifier a11. For example, the current signal isc is converted into a voltage and amplified with a resistance value of a feedback resistor rf1r in reproducing. On the other hand, in writing, the current signal isc is converted into a voltage and amplified with a parallel resistance value of a feedback resistor rf1w and the feedback resistor rf1r, which are connected parallel to each other in response to closing of a switch sw1. The feedback resistors rf1r and rf1w, which are provided to set gains, are adjusted to have resistance values that do not cause the amplifier a11 to be saturated in response to expected incident light power. The gain of the differential amplifier a21 is adjusted by an input resistor rs1 and a feedback resistor rf1 by their ratio rf1/rs1. The gain of the differential amplifier a22 is adjusted by an input resistor rs2 and a feedback resistor rf2 by their ratio rf2/rs2.
The first-stage amplifier 11 further includes a dummy amplifier a12, and feedback resistors rf2r and rf2w for the dummy amplifier a12. In addition, there is also provided a switch sw2, which closes with the switch sw1 in writing to connect the feedback resistor rf2w with the feedback resistor rf2r in parallel. The output of the dummy amplifier a12 is commonly supplied to inverted terminals of the differential amplifiers a21 and a22 of the second-stage amplifiers 12 and 13, respectively, so as to create their respective reference voltages.
This structure of the dummy amplifier a12 is needed when the amplifier a11 is a grounded amplifier and when it is connected to the differential amplifiers a21 and a22 of their respective second-stage amplifiers 12 and 13, which use the externally supplied reference power as reference voltages, because the reference voltages are determined by the internal amplifier structure in this case.
FIG. 12(a) through FIG. 12(d) are waveform diagrams explaining operations of the photo-detector amplifier circuit 11 having foregoing structure. An input optical signal shown in FIG. 12(a) is converted in the photodiode pd into the current signal isc shown in FIG. 12(b). The amplifier a11 outputs a signal that was created by current-voltage conversion of the signal shown in FIG. 12(c). FIG. 12(d) shows an output waveform that is created in the differential amplifier a21 by amplifying the result of comparison between the output of the amplifier a11 and an externally supplied reference voltage. The output waveform shown in FIG. 12(d) is also obtained, for example, when the outputs of the amplifiers a11 of the channel A through D are added and amplified in the differential amplifier a22. Note that, in FIG. 12(a) through FIG. 12(d), the broken lines with reference sign r indicate waveforms of reproducing, and solid lines with reference sign w indicate waveforms of writing.
In the recording and reproducing device 1, an optical system, including the beam splitter 4, is designed and fixed, taking into account the light of laser output (a quantity of light reflected into the photo-detector element A1) in writing and characteristics (sensitivity and/or dynamic range) of the photo-detector element A1. That is, a quantity of reflected light is adjusted so as not to cause the amplifier of the photo-detector element A1 to be saturated in response to a large quantity of light in writing. In addition to the adjustment in writing, the foregoing optical design also adjusts a quantity of reflected light in reproducing. This increases the speed of writing and, with increasing laser power, reduces the optical signal that enters the photo-detector element A1 in reproducing.
In this way, the photo-detector amplifier circuit 11 switches gains in response to a large light quantity in writing, so as to prevent the differential amplifier a11 from being saturated. However, with increasing laser power in response to a faster writing speed, gains tend to decrease both in writing and reproducing.
Read-only CD-ROM, writable CD-R, and rewritable CD-R/RW are some of the examples of the CD disk media that employs the laser light with a wavelength of 780 nm. Read-only DVD-ROM, writable DVD-R and DVD-RAM, and rewritable DVD-R/RW are some of the examples of DVD disk media that uses the laser light with a wavelength of 650 nm. Different disk types have different reflectances and the quantity of reflected laser light from the disk varies between these different types of disks. Generally, the rewritable CD-R/RW and DVD-R/RW disks have reflectances that are smaller by several factors than those of the read-only ROM disks.
The faster writing speed and diversification of the disk media have created a situation where the photo-detector amplifier circuit 11 receives a small optical signal when reproducing a low-reflective disk and there is a difficulty in reproducing signals from low-reflective disks.